Note: Descriptions are shown in the official language in which they were submitted.
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PROCESSES FOR REDUCING MIRO SAMINE FORMATION DURING GAS
PURIFICATION IN AMINE BASED LIQUID ABSORPTION SYSTEMS
CROSS REFERENCE TO RELA 11,D APPLICATIONS
This application claims the benefit of U. S. Provisional Application No.
61/469,233,
filed March 30, 2011, incorporated herein by reference in its entirety.
BACKGROUND
[0001] The present disclosure generally relates to processes for reducing
nitrosamine formation during gas purification of an acid gas stream containing
NOx with
amine based liquid absorption systems.
[0002] Power plants may combust various fuels, such as coal, hydrocarbons, bio-
mass, waste products, and the like, in boilers, for example, to generate steam
and
electricity. Exhaust streams (e.g., flue gas) of such combustion processes are
often treated
to neutralize or remove various compounds, such as carbon dioxide (CO2),
sulfur oxides,
nitrogen oxides (N0x), and particulate matter, prior to discharge of the flue
gas to the
environment. These downstream processes include, among others, post-combustion
capture systems. The challenge here is the large volume of the flue gas due to
essentially
atmospheric pressure and the presence of N2. The CO2 contents are also
relatively small
which leads to veiy large equipment for the capture section.
[0003] In post-combustion processes used for separation of acidic gases such
as
CO2 from a flue gas stream, liquid solutions comprising amine compounds or
aqueous
ammonia solutions are commonly used as a wash solution. The acidic gases are
absorbed
by the amine based wash solution in an absorption unit to form a soluble salt
solution
referred to as a rich amine solution containing the absorbed acid gas in an
absorption
process, e.g., a bicarbonate salt. The absorbed acid gas in the form of the
salt is then
desorbed or stripped from the amine based solvent, generally at a higher
temperature
and/or change in pressure, in a regeneration unit.
[0004] The ability of the amine based solvent to remove carbon dioxide is
generally dependent on its equilibrium solubility as well as mass transfer and
chemical
kinetics characteristics. Exemplary amine compounds utilized for the amine
based wash
solution generally include monoethanolamine (MEA), diethanolamine (DEA),
methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and aminoethoxyethanol
(diglycolamine), 2-amino-2-methyl- 1 -propanol (AMP) and various combinations
thereof
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The amine based wash solution may further include a promoter and/or a
catalyst. The
promoters and/or catalysts are generally utilized to enhance the reaction
kinetics involved
in the capture of CO2. Exemplaiy promoters and catalysts include a secondary
diamine or
triamine such as piperazine or enzymes such as carbonic anhydrase or its
analogs. The
promoters may be in the form of a solution or immobilized on solid or
semisolid surfaces.
Inhibitors are generally provided to minimize corrosion and solvent
degradation.
[0005] In amine based wash systems that employ secondary amines such as
piperazine, nitrosamines can be formed if the acid gas stream contains NOx.
The NOx,
which may include NO, NO2, N203, and solution reaction products such as NO2-
interact
with the secondary amines to form the nitrosamines. Nitrosamines are
considered
hazardous and may need special handling and/or off gas treatment.
[0006] In view of the foregoing, there is a need in the art to reduce
nitrosainine
formation during gas purification of gas streams that contain NOx with amine
based liquid
absorption systems.
BRIEF SUMMARY
[0007] Disclosed herein are acid gas purification processes for reducing
nitrosamine precursor formation from a gas stream containing NOx. In one
embodiment,
an acid gas purification process for reducing nitrosamine precursor formation
from a gas
stream containing NOx, wherein the acid gas is selectively absorbed in an
amine based
wash solution comprising at least one secondaiy diamine or triamine comprises
absorbing
carbon dioxide from the gas stream containing NOx species with the amine-based
wash
solution comprising at least one secondary diamine or triamine to provide a
gas stream free
of carbon dioxide that is released into the surroundings, wherein absorbing
the acid gas
forms a rich amine solution; and regenerating the rich amine solution at an
elevated
temperature to release the carbon dioxide to form a regenerated lean amine
solution,
wherein absorbing and regenerating are configured to promote formation of
carbamate
species of the at least one diamine or triamine relative to bicarbonate
species.
[0008] In another embodiment, the acid gas purification process for reducing
nitrosamine precursor formation from a gas stream containing NOx, wherein the
acid gas
is selectively absorbed in an amine based wash solution comprising at least
one secondary
diamine or triamine, the process comprises absorbing carbon dioxide from the
gas stream
containing NOx species with the amine-based wash solution comprising at least
one
secondary diamine or triamine to provide a carbon dioxide lean gas stream that
is released
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into the surroundings, wherein absorbing the acid gas forms a rich amine
solution;
regenerating the rich amine solution at an elevated temperature to release the
carbon
dioxide to form a regenerated lean amine solution; and removing heat stable
amine salts to
less than 1%.
[0009] The disclosure may be understood more readily by reference to the
following detailed description of the various features of the disclosure and
the examples
included therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Referring now to the figures wherein the like elements are numbered
alike:
[0011] FIG. 1 depicts an exemplary liquid amine absorption system for removing
acid gases from a gas stream;
[0012] FIG. 2 graphically illustrates a prior art plot of log (k2 in M-1 sec-
1) against
the pKa for the nitrosation of various secondary amines; and
[0013] FIG. 3 graphically illustrates predicted species distribution as a
function of
CO2 loading in a CO2/piperazine activated MDEA amine based solvent system.
DETAILED DESCRIPTION
[0014] Disclosed herein are processes for reducing nitrosamine formation
during
gas purification of acid gas streams that include NOx with amine based liquid
absorption
systems that include secondary diamines or triamines. The applicant has
discovered that
controlling basicity during gas purification can minimize formation of
nitrosamine
precursors during gas purification. As will be discussed in greater detail
herein,
controlling basicity can generally be accomplished by minimizing forination of
heat stable
salts and/or controlling rich amine loading.
[0015] Referring now to FIG. 1, a typical gas purification system, generally
designated by reference numeral 10, includes an absorption unit 12 and a
regeneration unit
14. The absorption and regeneration units 12, 14 may be a column such as a
packed bed
column or a column containing trays. The absorption unit 12 is arranged to
allow contact
between a gas stream to be purified and one or more amine based wash liquids.
The
absorption unit generally includes an amine wash section 16 for CO2 absorption
and a
water wash section 18 for contaminant removal. Intermediate to sections 16 and
18 there
may be a condenser 20.
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[0016] Flue gas from which CO2 is to be removed is fed to a lower portion of
the
absorption unit 12 via line 22. In section 16, the flue gas is contacted in
countercurrent
fashion with a wash liquid comprising an amine wash liquid, e.g., by bubbling
the flue gas
through the wash liquid or by spraying the wash liquid into the flue gas. The
amine wash
liquid is fed to the absorption unit via line 24. CO2 from the flue gas is
absorbed in the
amine wash liquid and is discharged from the absorption unit via line 32. The
dissolved
CO2 forms carbonic acid and products of its deprotonation, which react with
the amine
based solvent system. In addition, promoters may form amine carbamic acid and
its salts.
Flue gas substantially depleted of CO2 in the absorption section 16 then
enters the water
wash section 18, wherein the flue gas contacts a second wash liquid, which is
generally
water, for removing water soluble contaminants from the flue gas. The second
wash liquid
is fed to the absorption unit via line 26.
[0017] The wash water utilized in wash water section 18 is generated self
sufficient
by condensing part of the water vapor contained in the treated gas coming from
the CO2
absorption section 16. Excess water is not discharged as an effluent but
instead is sent to
the amine wash solution loop via line 28. Flue gas depleted of CO2 and
contaminants
leaves the absorption unit via line 30 and may be discharged to the
atmosphere. The used
first and second wash liquids containing absorbed CO2 and contaminants leave
the
absorption unit via line 32, which is commonly referred to as the rich amine.
[0018] The used first and second wash liquids are recycled by pumping the rich
amine solution to the regenerator unit 14, wherein the acid gases such as CO2
are then
stripped from the wash liquids. A portion of the rich amine solution may be
heated via
heat exchanger 34 and fed to a mid-section of the regenerator (e.g., which may
be at about
100 to 150 C) or fed to the top portion of the regenerator unit 14, which can
be at a
markedly lower temperature so as to minimize the energy losses due to the
latent heat of
the water vapor (e.g., typically 40 to 60 C). The rich amine wash solution is
withdrawn
from the lower section and provided to a reboiler 36 positioned downstream of
the
regenerator. There the CO2 is stripped at a relatively high temperature and
leaves the
system via line 38.
[0019] The reboiler 36 boils the rich amine solution to form steam and a hot
regenerated wash solution (i.e., lean amine solution), which is recycled for
use in the
absorption unit 12 via line 40. The heating of the regeneration unit from the
bottom gives
a temperature gradient at steady state from the bottom to the top, wherein the
top portion
of the regeneration unit is lower relative to the bottom depending on the
configuration.
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[0020] To take advantage of the thermal energy present in the lean amine
solution
as it exits the regeneration unit 14, the hot regenerated wash solution can be
first directed
to heat exchanger 34, where it exchanges heat and is cooled relative to the
incoming rich
amine solution from the absorption unit, which is heated. The lean amine
solution is
typically at a temperature of about 120 C whereas the rich amine solution is
at a
temperature of about 90 to 110 C when these solutions exit heat exchanger 34.
The lean
amine solution may be further cooled in a cooler, if desired, prior to use in
the absorption
unit 12.
[0021] The process description above is intended to represent a general
description
of an amine scrubber in order to illustrate the concept of the invention. It
should be
apparent to those skilled in the art that other process flow schemes
including, but not
limited to, multiple absorbers and strippers, intermediate cooling steps and
alternate
temperatures and pressures may be utilized.
[0022] For ease of understanding, reference will now be made to piperazine
activated MDEA amine based solvent systems. Piperazine is a cyclic diamine
bearing two
secondary amine groups, and when used, functions as a promoter to the MDEA.
Theoretically, piperazine can bind with 2 mol of CO2. Piperazine reacts
rapidly and
strongly binds with CO2 It then shuttles the CO2 as the carbamate into the
interior of the
liquid. Nitrosamines can be formed when NOx from the acidic gas stream to be
treated or
NOx products of its reaction with water interact in solution with primary and
secondary
amines such as piperazine. However, the present disclosure is not intended to
be limited to
piperazine activated MDEA amine based solvent systems and is generally
applicable to
any amine based solvent system that includes a secondary diamine or triamine
either as the
base solvent or as the promoter. Other secondary diamines and triamines that
could be
used include, without limitation 1-methylpiperazine, 2-methylpiperazine, N-
methylethylenediamine, diethylenetriamine,a nd mixtures thereof.
[0023] Referring now to FIG. 2, there is graphically shown a prior art plot of
log
(k2 in M-1 sec-1) against the pKa for the nitrosation of various secondary
amines. As
shown, amines with low pKa (low basicity) will form nitrosamines faster than
highly basic
amines. This is consistent with the high reactivity of piperazine being
attributed to the
second less basic nitrogen. When unreacted, both amine groups are equivalent
and exhibit
relatively high basicity, e.g., pKa = 9.7. However, when monoprotonated, the
basicity of
the second amine group drops to a pKa of about 5.6 and reacts many times
faster with
nitrosating agents to form nitrosopiperazine. For instance, from Figure 2 it
can be shown
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that the rate of nitrosamine formation is greater by a factor of about 10,000
for reaction of
the monoprotonated piperazine.
Accordingly, minimizing formation of the
monoprotonated piperazine can lead to a significant reduction in nitrosamine
formation,
which may be accomplished by minimizing formation of heat stable salts, which
increases
acidity of the solvent thereby promoting piperazine protonation, and/or
lowering acid gas
loading, which, for similar reasons, will minimize formation of the
monoprotonated
piperazine species.
[0024] Reaction of CO2 with piperazine generally occurs via two reaction
pathways: a) formation of a bicarbonate salt, and b) formation of the
piperazine carbamate
(and the dicarbamate). FIG. 3 shows how variation of the split of
bicarbonate/carbamate
changes the concentration of nitrosamine precursor by graphically illustrating
predicted
species distribution as a function of CO2 loading in a CO2/piperazine
activated MDEA
system. As shown, adjustment of CO2 loading can effectively reduce the
concentration of
monoprotonated piperazine, which should result in reducing the formation rate
of
nitrosamine. For instance, at the conditions at which Figure 3 is based, the
concentration
of monoprotonated piperazine (PIPH34) reaches a maximum value at a loading of
about
1.2 (mCO2/mol-kg) At a
loading of only 1/2 that value, the concentration of
monoprotonated piperazine (and hence the reaction rate for nitrosamines) is
only 1/2 of the
maximum rate.
[0025] Accordingly, in one embodiment, the process for reducing nitrosamine
formation includes decreasing the rich amine loading. For example, rich amine
loading
can be decreased by decreasing the amount of piperazine in the amine based
wash solution
and by decreasing residence time in the absorber unit. By reducing the amount
of
piperazine in the amine based wash solution and/or reducing residence time,
the amount of
absorbed CO2 will decrease. Reduction of the CO2 loading by a relatively small
amount
can have a significant effect on reducing nitrosamine formation. One skilled
in the art can
easily optimize nitrosamine formation with solution loading, wherein the
specific ranges
will vary for different solvent compositions and process conditions.
[0026] In another embodiment, the absorption unit 12 can be configured to
provide
a shorter pathway and/or the flow rate configured to reduce the residence time
of the flue
gas in the absorption unit. Doing so will maximize formation of carbamate
species while
minimizing bicarbonate production.
[0027] Still further, another embodiment includes minimizing formation of heat
stable salts. When the amine based solvent comes into contact with the flue-
gases, the
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amines will also react with other contaminants in the flue-gas, such as S02,
02, NOx, and
the like. How much of these are absorbed will vary from one amine to another,
and will
also depend on the design of the absorption unit. These reactions can form
heat-stable
salts that are non-regenerable under solvent regeneration conditions, i.e.,
will not be
released from the amine solution by the steam stripping process in the
regeneration unit.
In addition, these reactions can form acids. For example, oxygen can react
with the other
components to form oxalic acid, acetic acid, formic acid, and the like. Since
the amine
mixture is circulated between the absorber and the desorber, the amount of
heat-stable salts
in the solvent will gradually rise. After a certain period of time, the
concentration of these
salts will be so high that the CO2 absorption rate will be reduced. This is
handled by the
use of a reclaiming unit. Applicant has discovered that higher levels of
the
nitrosopiperazine precursor are formed as a function of increasing amounts of
heat stable
salts. In this embodiment, the system can be configured to minimize production
of heat
stable salts. In one embodiment, the reclaiming unit is configured to maintain
the heat
stable salt concentration at less than 1%. In other embodiment, the reclaiming
unit is
configured to maintain the heat stable salt concentration at less than 0.5%.
[0028] Advantageously, the present invention reduces formation of nitrosamine
precursors.
[0029] Unless otherwise specified, all ranges disclosed herein are inclusive
and
combinable at the end points and all intermediate points therein. The terms
"first,"
"second," and the like, herein do not denote any order, quantity, or
importance, but rather
are used to distinguish one element from another. The terms "a" and "an"
herein do not
denote a limitation of quantity, but rather denote the presence of at least
one of the
referenced item. All numerals modified by "about" are inclusive of the precise
numeric
value unless otherwise specified.
[0030] Variations, modifications, and other implementations of what is
described
may be employed without departing from the spirit and scope of the invention.
More
specifically, any of the method, system and device features described above or
incorporated by reference may be combined with any other suitable method,
system or
device features disclosed herein or incorporated by reference, and is within
the scope of
the contemplated inventions. The systems and methods may be embodied in other
specific
forms without departing from the spirit or essential characteristics thereof.
The foregoing
embodiments are therefore to be considered in all respects illustrative,
rather than limiting
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of the invention. The teachings of all references cited herein are hereby
incorporated by
reference in their entirety.
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